Aircraft having redundant directional control

ABSTRACT

An aircraft has an airframe with a two-dimensional distributed thrust array attached thereto having a plurality of propulsion assemblies that are independently controlled by a flight control system. Each propulsion assembly includes a housing with a gimbal coupled thereto that is operable to tilt about first and second axes responsive to first and second actuators. A propulsion system is coupled to and operable to tilt with the gimbal. The propulsion system includes an electric motor having an output drive and a rotor assembly having a plurality of rotor blades that rotate in a rotational plane to generate thrust having a thrust vector. Responsive to a thrust vector error of a first propulsion assembly, the flight control system commands at least a second propulsion assembly, that is symmetrically disposed relative to the first propulsion assembly, to counteract the thrust vector error, thereby providing redundant directional control for the aircraft.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of co-pendingapplication Ser. No. 15/606,242 filed May 26, 2017, which is acontinuation-in-part of application Ser. No. 15/200,163 filed Jul. 1,2016 and the present application claims the benefit of U.S. ProvisionalApplication No. 62/594,424, filed Dec. 4, 2017, the entire contents ofeach is hereby incorporated by reference.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to aircraft operable totransition between thrust-borne flight and wing-borne flight and, inparticular, to aircraft having a distributed thrust array including aplurality of propulsion assemblies each having a gimbal mountedpropulsion system operable for thrust vectoring.

BACKGROUND

Fixed-wing aircraft, such as airplanes, are capable of flight usingwings that generate lift responsive to the forward airspeed of theaircraft, which is generated by thrust from one or more jet engines orpropellers. The wings generally have an airfoil cross section thatdeflects air downward as the aircraft moves forward, generating the liftforce to support the airplane in flight. Fixed-wing aircraft, however,typically require a runway that is hundreds or thousands of feet longfor takeoff and landing.

Unlike fixed-wing aircraft, vertical takeoff and landing (VTOL) aircraftdo not require runways. Instead, VTOL aircraft are capable of takingoff, hovering and landing vertically. One example of VTOL aircraft is ahelicopter which is a rotorcraft having one or more rotors that providelift and thrust to the aircraft. The rotors not only enable hovering andvertical takeoff and landing, but also enable, forward, backward andlateral flight. These attributes make helicopters highly versatile foruse in congested, isolated or remote areas where fixed-wing aircraft maybe unable to takeoff and land. Helicopters, however, typically lack theforward airspeed of fixed-wing aircraft.

A tiltrotor aircraft is another example of a VTOL aircraft. Tiltrotoraircraft generate lift and propulsion using proprotors that aretypically coupled to nacelles mounted near the ends of a fixed wing. Thenacelles rotate relative to the fixed wing such that the proprotors havea generally horizontal plane of rotation for vertical takeoff, hoveringand landing and a generally vertical plane of rotation for forwardflight, wherein the fixed wing provides lift and the proprotors provideforward thrust. In this manner, tiltrotor aircraft combine the verticallift capability of a helicopter with the speed and range of fixed-wingaircraft. Tiltrotor aircraft, however, typically suffer from downwashinefficiencies during vertical takeoff and landing due to interferencecaused by the fixed wing.

A further example of a VTOL aircraft is a tiltwing aircraft thatfeatures a rotatable wing that is generally horizontal for forwardflight and rotates to a generally vertical orientation for verticaltakeoff and landing. Propellers are coupled to the rotating wing toprovide the required vertical thrust for takeoff and landing and therequired forward thrust to generate lift from the wing during forwardflight. The tiltwing design enables the slipstream from the propellersto strike the wing on its smallest dimension, thus improving verticalthrust efficiency as compared to tiltrotor aircraft. Tiltwing aircraft,however, are more difficult to control during hover as the verticallytilted wing provides a large surface area for crosswinds typicallyrequiring tiltwing aircraft to have either cyclic rotor control or anadditional thrust station to generate a moment.

SUMMARY

In a first aspect, the present disclosure is directed to an aircrafthaving redundant directional control. The aircraft has an airframe witha two-dimensional distributed thrust array attached thereto. The thrustarray includes a plurality of propulsion assemblies each of which isindependently controlled by a flight control system. Each propulsionassembly includes a housing having a gimbal coupled thereto that isoperable to tilt about first and second axes. First and second actuatorsare operable to tilt the gimbal respectively about the first and secondaxes. A propulsion system is coupled to and is operable to tilt with thegimbal. The propulsion system includes an electric motor having anoutput drive and a rotor assembly having a plurality of rotor blades.The rotor assembly is rotatable with the output drive of the electricmotor in a rotational plane to generate thrust having a thrust vector.Responsive to a thrust vector error of a first propulsion assembly, theflight control system commands at least a second propulsion assembly,that is symmetrically disposed relative to the first propulsionassembly, to counteract the thrust vector error, thereby providingredundant directional control for the aircraft.

In certain embodiments, the flight control system may be a redundantflight control system or a triply redundant flight control system. Insome embodiments, the commands of the flight control system to thesecond propulsion assembly may include tilting the second propulsionassembly about the first axis, tilting the second propulsion assemblyabout the second axis, changing the operating speed of the rotorassembly of the second propulsion assembly and combinations thereof. Incertain embodiments, responsive to the thrust vector error of the firstpropulsion assembly, the flight control system may command at least twoother propulsion assemblies to counteract the thrust vector error,thereby providing redundant directional control for the aircraft.

In some embodiments, when the thrust vector error of the firstpropulsion assembly is a static actuator fault causing the propulsionsystem of the first propulsion assembly to cease tilting about one axis,the flight control system may command the second propulsion assembly tocounteract the single-axis static actuator fault. In certainembodiments, when the thrust vector error of the first propulsionassembly is a static actuator fault causing the propulsion system of thefirst propulsion assembly to cease tilting about both axes, the flightcontrol system may command the second propulsion assembly to counteractthe two-axis static actuator fault. In some embodiments, when the thrustvector error of the first propulsion assembly is a dynamic actuatorfault causing the propulsion system of the first propulsion assembly totilt uncontrolled about one axis, the flight control system may commandthe second propulsion assembly to counteract the single-axis dynamicactuator fault. In certain embodiments, when the thrust vector error ofthe first propulsion assembly is a dynamic actuator fault causing thepropulsion system of the first propulsion assembly to tilt uncontrolledabout both axes, the flight control system may command the secondpropulsion assembly to counteract the two-axis dynamic actuator fault.

In some embodiments, responsive to the thrust vector error of the firstpropulsion assembly, the flight control system may command the aircraftto land at a predetermined location, to perform an emergency landing, tocontinue a current mission, to adjust a center of mass of a payloadrelative to the airframe, to initiate a jettison sequence or somecombination thereof. In certain embodiments, the aircraft may include atleast four propulsion assemblies forming the two-dimensional thrustarray. In some embodiments, the aircraft may have a thrust-borne flightmode and a wing-borne flight mode. In certain embodiments, the airframemay include first and second wings having at least first and secondpylons extending therebetween and having a plurality of tail membersextending therefrom, each tail member having a control surface. In someembodiments, the aircraft may include a pod assembly coupled to theairframe.

In a second aspect, the present disclosure is directed to an aircrafthaving a thrust-borne flight mode and a wing-borne flight mode. Theaircraft includes an airframe having first and second wings with atleast first and second pylons extending therebetween and with aplurality of tail members extending therefrom. A pod assembly is coupledto the airframe between the first and second pylons. A two-dimensionaldistributed thrust array is attached to the airframe. The thrust arrayincludes at least four line replaceable propulsion units each of whichis independently controlled by a flight control system. Each propulsionunit includes a housing with and a gimbal coupled thereto that isoperable to tilt about first and second axes. First and second actuatorsare operable to tilt the gimbal respectively about the first and secondaxes. A propulsion system is coupled to and is operable to tilt with thegimbal. The propulsion system includes an electric motor having anoutput drive and a rotor assembly having a plurality of rotor blades.The rotor assembly is rotatable with the output drive of the electricmotor in a rotational plane to generate thrust having a thrust vector.Responsive to a thrust vector error of a first propulsion assembly, theflight control system commands at least a second propulsion assembly,that is symmetrically disposed relative to the first propulsionassembly, to counteract the thrust vector error, thereby providingredundant directional control for the aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIGS. 1A-1F are schematic illustrations of an aircraft in accordancewith embodiments of the present disclosure;

FIGS. 2A-2I are schematic illustrations of an aircraft in a sequentialflight operating scenario in accordance with embodiments of the presentdisclosure;

FIG. 3 is a block diagram of a two-dimensional distributed thrust arrayhaving two-axis gimbal mounted propulsion systems for an aircraft inaccordance with embodiments of the present disclosure;

FIGS. 4A-4D are schematic illustrations of an aircraft performingvarious flight maneuvers in accordance with embodiments of the presentdisclosure;

FIGS. 5A-5I are schematic illustrations of a line replaceable propulsionunit operating a two-axis gimbal for an aircraft in accordance withembodiments of the present disclosure;

FIGS. 6A-6D are schematic illustrations of an aircraft performingmeasures to counteract an actuator fault in a propulsion assembly inaccordance with embodiments of the present disclosure;

FIG. 7 is a block diagram of a two-dimensional distributed thrust arrayhaving single-axis gimbal mounted propulsion systems for an aircraft inaccordance with embodiments of the present disclosure;

FIGS. 8A-8B are schematic illustrations of an aircraft performingvarious flight maneuvers in accordance with embodiments of the presentdisclosure;

FIGS. 9A-9C are schematic illustrations of a line replaceable propulsionunit operating a single-axis gimbal for an aircraft in accordance withembodiments of the present disclosure; and

FIGS. 10A-10D are schematic illustrations of a tail member having acontrol surface and a line replaceable propulsion unit coupled theretofor an aircraft in accordance with embodiments of the presentdisclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,not all features of an actual implementation may be described in thepresent disclosure. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would be a routine undertakingfor those of ordinary skill in the art having the benefit of thisdisclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicedescribed herein may be oriented in any desired direction. As usedherein, the term “coupled” may include direct or indirect coupling byany means, including moving and/or non-moving mechanical connections.

Referring to FIGS. 1A-1F in the drawings, various views of an aircraft10 having a distributed thrust array including gimbal mounted propulsionsystems operable for thrust vectoring are depicted. FIGS. 1A, 1C, 1Edepict aircraft 10 in thrust-borne flight which may also be referred toas the vertical takeoff and landing or VTOL flight mode of aircraft 10.FIGS. 1B, 1D, 1F depict aircraft 10 in wing-borne flight which may alsobe referred to as the forward or high speed forward flight mode ofaircraft 10. In the illustrated embodiment, the airframe 12 of aircraft10 includes wings 14 a, 14 b each having an airfoil cross-section thatgenerates lift responsive to the forward airspeed of aircraft 10. Wings14 a, 14 b may be formed as single members or may be formed frommultiple wing sections. The outer skins for wings 14 a, 14 b arepreferably formed from high strength and lightweight materials such asfiberglass, carbon, plastic, metal or other suitable material orcombination of materials. As illustrated, wings 14 a, 14 b are straightwings. In other embodiments, wings 14 a, 14 b could have other designssuch as polyhedral wing designs, swept wing designs or other suitablewing design.

Extending generally perpendicularly between wings 14 a, 14 b are twotruss structures depicted as pylons 16 a, 16 b. In other embodiments,more than two pylons may be present. Pylons 16 a, 16 b are preferablyformed from high strength and lightweight materials such as fiberglass,carbon, plastic, metal or other suitable material or combination ofmaterials. Wings 14 a, 14 b and pylons 16 a, 16 b may be coupledtogether at the respective intersections using mechanical connectionssuch as bolts, screws, rivets, adhesives and/or other suitable joiningtechnique. Extending generally perpendicularly from wings 14 a, 14 b arelanding gear depicted as tail members 18 a, 18 b, 18 c, 18 d that enableaircraft 10 to operate as a tailsitting aircraft. In the illustratedembodiment, tail members 18 a, 18 b, 18 c, 18 d are fixed landingstruts. In other embodiments, tail members 18 a, 18 b, 18 c, 18 d mayinclude passively operated pneumatic landing struts or actively operatedtelescoping landing struts with or without wheels for ground maneuvers.Tail members 18 a, 18 b, 18 c, 18 d each include a control surface 20 a,20 b, 20 c, 20 d, respectively, that may be passive or activeaerosurfaces that serve as vertical stabilizers and/or elevators duringwing-borne flight and serve to enhance hover stability duringthrust-borne flight.

Wings 14 a, 14 b and pylons 16 a, 16 b preferably include centralpassageways operable to contain flight control systems, energy sources,communication lines and other desired systems. For example, as best seenin FIG. 1A, wing 14 a houses the flight control system 32 of aircraft10. Flight control system 32 is preferably a redundant digital flightcontrol system. In the illustrated embodiment, flight control system 32is a triply redundant digital flight control system including threeindependent flight control computers. Use of triply redundant flightcontrol system 32 having redundant components improves the overallsafety and reliability of aircraft 10 in the event of a failure inflight control system 32. Flight control system 32 preferably includesnon-transitory computer readable storage media including a set ofcomputer instructions executable by one or more processors forcontrolling the operation of aircraft 10. Flight control system 32 maybe implemented on one or more general-purpose computers, special purposecomputers or other machines with memory and processing capability. Forexample, flight control system 32 may include one or more memory storagemodules including, but is not limited to, internal storage memory suchas random access memory, non-volatile memory such as read only memory,removable memory such as magnetic storage memory, optical storage,solid-state storage memory or other suitable memory storage entity.Flight control system 32 may be a microprocessor-based system operableto execute program code in the form of machine-executable instructions.In addition, flight control system 32 may be selectively connectable toother computer systems via a proprietary encrypted network, a publicencrypted network, the Internet or other suitable communication networkthat may include both wired and wireless connections.

In the illustrated embodiment, wings 14 a, 14 b and/or pylons 16 a, 16 bmay contain one or more of electrical power sources depicted asbatteries 22 in wing 14 a, as best seen in FIG. 1A. Batteries 22 supplyelectrical power to flight control system 32. In some embodiments,batteries 22 may be used to supply electrical power for the distributedthrust array of aircraft 10. Wings 14 a, 14 b and/or pylons 16 a, 16 balso contain a communication network 24 that enables flight controlsystem 32 to communicate with the distributed thrust array of aircraft10.

In the illustrated embodiment, the distributed thrust array includesfour propulsion assemblies 26 a, 26 b, 26 c, 26 d that are independentlyoperated and controlled by flight control system 32. It should be noted,however, that the distributed thrust array of the present disclosurecould have any number of independent propulsion assemblies includingsix, eight, twelve, sixteen or other number of independent propulsionassemblies. Propulsion assemblies 26 a, 26 b, 26 c, 26 d areindependently attachable to and detachable from airframe 12. Forexample, propulsion assemblies 26 a, 26 b, 26 c, 26 d are preferablystandardized and interchangeable units that are most preferably linereplaceable propulsion units enabling easy installation and removal fromairframe 12. Propulsion assemblies 26 a, 26 b, 26 c, 26 d may be coupledto wings 14 a, 14 b using quick connect and disconnect couplingstechniques including bolts, pins, cables or other suitable couplingtechniques. In addition, the use of line replaceable propulsion units isbeneficial in maintenance situations if a fault is discovered with oneof the propulsion units. In this case, the faulty propulsion unit can bedecoupled from airframe 12 by simple operations and another propulsionunit can then be attached to airframe 12. In other embodiments,propulsion assemblies 26 a, 26 b, 26 c, 26 d may be permanently coupledto wings 14 a, 14 b by riveting, bonding and/or other suitabletechnique.

As illustrated, propulsion assemblies 26 a, 26 b, 26 c, 26 d are coupledto the outboard ends of wings 14 a, 14 b. In other embodiments,propulsion assemblies 26 a, 26 b, 26 c, 26 d could have otherconfigurations including close coupled configurations, high wingconfigurations, low wing configurations or other suitable configuration.In the illustrated embodiment, the four independently operatingpropulsion assemblies 26 a, 26 b, 26 c, 26 d form a two-dimensionalthrust array with each of the propulsion assemblies having asymmetrically disposed propulsion assembly. For example, propulsionassemblies 26 a, 26 c are symmetrically disposed propulsion assembliesand propulsion assemblies 26 b, 26 d are symmetrically disposedpropulsion assemblies. It should be noted, however, that atwo-dimensional thrust array of the present disclosure could have anynumber of independent propulsion assemblies including six, eight,twelve, sixteen or other number of independent propulsion assembliesthat form the two-dimensional thrust array with each of the propulsionassemblies having a symmetrically disposed propulsion assembly.

In the illustrated embodiment, each propulsion assembly 26 a, 26 b, 26c, 26 d includes a housing 28 a, 28 b, 28 c, 28 d, that containscomponents such as an electric motor, a gimbal, one or more actuatorsand an electronics node including, for example, batteries, controllers,sensors and other desired electronic equipment. Only electric motors 30a, 30 b and electronics nodes 32 a, 32 b are visible in FIG. 1A. Theelectric motors of each propulsion assembly 26 a, 26 b, 26 c, 26 d arepreferably operated responsive to electrical energy from the battery orbatteries disposed with that housings, thereby forming a distributedelectrically powered thrust array. Alternatively or additionally,electrical power may be supplied to the electric motors and/or thebatteries disposed with the housing from batteries 22 carried byairframe 12 via communications network 24. In other embodiments, thepropulsion assemblies may include internal combustion engines orhydraulic motors.

Flight control system 32 communicates via communications network 24 withthe electronics nodes of each propulsion assembly 26 a, 26 b, 26 c, 26d, such as electronics node 32 a of propulsion assembly 26 a andelectronics node 32 b of propulsion assembly 26 b. Flight control system32 receives sensor data from and sends flight command information to theelectronics nodes of each propulsion assembly 26 a, 26 b, 26 c, 26 dsuch that each propulsion assembly 26 a, 26 b, 26 c, 26 d may beindividually and independently controlled and operated. For example,flight control system 32 is operable to individually and independentlycontrol the operating speed and thrust vector of each propulsionassembly 26 a, 26 b, 26 c, 26 d. Flight control system 32 mayautonomously control some or all aspects of flight operation foraircraft 10. Flight control system 32 is also operable to communicatewith remote systems, such as a ground station via a wirelesscommunications protocol. The remote system may be operable to receiveflight data from and provide commands to flight control system 32 toenable remote flight control over some or all aspects of flightoperation for aircraft 10. The autonomous and/or remote operation ofaircraft 10 enables aircraft 10 to perform unmanned logistic operationsfor both military and commercial applications.

Each propulsion assembly 26 a, 26 b, 26 c, 26 d includes a rotorassembly 34 a, 34 b, 34 c, 34 d. Each rotor assembly 34 a, 34 b, 34 c,34 d is directly or indirectly coupled to an output drive of arespective electrical motor 30 a, 30 b, 30 c, 30 d that rotates therotor assembly 34 a, 34 b, 34 c, 34 d in a rotational plane to generatethrust for aircraft 10. In the illustrated embodiment, rotor assemblies34 a, 34 b, 34 c, 34 d each include three rotor blades having a fixedpitch. In other embodiments, the rotor assemblies could have othernumbers of rotor blades both less than and greater than three.Alternatively or additionally, the rotor assemblies could have variablepitch rotor blades with collective and/or cyclic pitch control. Eachelectrical motor 30 a, 30 b, 30 c, 30 d is paired with a rotor assembly34 a, 34 b, 34 c, 34 d, for example electrical motor 30 a and rotorassembly 34 a, to form a propulsion system 36 a, 36 b, 36 c, 36 d. Asdescribed herein, each propulsion system 36 a, 36 b, 36 c, 36 d may havea single-axis or a two-axis tilting degree of freedom relative tohousings 28 a, 28 b, 28 c, 28 d and thus airframe 12 such thatpropulsion systems 36 a, 36 b, 36 c, 36 d are operable for thrustvectoring. In the illustrated embodiment, the maximum angle of thethrust vector may preferably be between about 10 degrees and about 30degrees, may more preferably be between about 15 degrees and about 25degrees and may most preferably be about 20 degrees. Notably, using a20-degree thrust vector yields a lateral component of thrust that isabout 34 percent of total thrust. In other embodiments, the propulsionsystems may not have a tilting degree of freedom in which case,propulsion systems 36 a, 36 b, 36 c, 36 d may not be capable of thrustvectoring. As such, aircraft 10 may have no thrust vectoringcapabilities, single-axis thrust vectoring capabilities or two-axisthrust vectoring capabilities associated with each propulsion assembly26 a, 26 b, 26 c, 26 d.

Aircraft 10 may operate as a transport aircraft for a pod assembly 50that is fixed to or selectively attachable to and detachable fromairframe 12. In the illustrated embodiment, pylons 16 a, 16 b includereceiving assemblies for coupling with pod assembly 50. The connectionbetween pylons 16 a, 16 b and pod assembly 50 may be a fixed connectionthat secures pod assembly 50 in a single location relative to airframe12. Alternatively, pod assembly 50 may be allowed to rotate and/ortranslate relative to airframe 12 during ground and/or flightoperations. For example, it may be desirable to have pod assembly 50 lowto the ground for loading and unloading cargo but more distant from theground for takeoff and landing. As another example, it may be desirableto change the center of mass of pod assembly 50 relative to airframe 12during certain flight conditions such as moving the center of mass ofpod assembly 50 forward relative to airframe 12 during high speedwing-borne flight. Similarly, it may be desirable to lowering the centerof mass of pod assembly 50 relative to airframe 12 during hover in theevent of a partial or total failure of one of the propulsion assemblies.As illustrated, pod assembly 50 may be selectively coupled to anddecoupled from airframe 12 to enable sequential pickup, transportationand delivery of multiple pod assemblies 50 to and from multiplelocations.

Airframe 12 preferably has remote release capabilities of pod assembly50. For example, this feature allows airframe 12 to drop pod assembly 50at a desire location following transportation. In addition, this featureallows airframe 12 to jettison pod assembly 50 during flight, forexample, in the event of an emergency situation such as a propulsionassembly or other system of aircraft 10 becoming compromised. One ormore communication channels may be established between pod assembly 50and airframe 12 when pod assembly 50 is attached therewith. A quickdisconnect harness may be coupled between pod assembly 50 and airframe12 such that flight control system 32 may send commands to pod assembly50 to perform functions. For example, flight control system 32 mayoperate doors of pod assembly 50 between open and closed positions toenable loading and unloading of a payload to be transported within podassembly 50.

Referring additionally to FIGS. 2A-2I in the drawings, a sequentialflight-operating scenario of aircraft 10 is depicted. In the illustratedembodiment, pod assembly 50 is attached to airframe 12 and may contain adesired payload. It is noted, however, that pod assembly 50 may beselectively disconnected from airframe 12 such that a single airframecan be operably coupled to and decoupled from numerous pod assembliesfor numerous missions over time. In addition, aircraft 10 may performmissions without having a pod assembly attached to airframe 12. As bestseen in FIG. 2A, aircraft 10 is in a tailsitting position on the ground.When aircraft 10 is ready for a mission, flight control system 32commences operations to provide flight control to aircraft 10 which maybe autonomous flight control, remote flight control or a combinationthereof. For example, it may be desirable to utilize remote flightcontrol during certain maneuvers such as takeoff and landing but rely onautonomous flight control during hover, high speed forward flight and/ortransitions between wing-borne flight and thrust-borne flight.

As best seen in FIG. 2B, aircraft 10 has performed a vertical takeoffand is engaged in thrust-borne flight with pod assembly 50 lifted intothe air. As illustrated, the rotor assemblies 34 a, 34 b, 34 c, 34 d areeach rotating in the same horizontal plane forming of a two-dimensionaldistributed thrust array. As noted, flight control system 32independently controls and operates each propulsion assembly 26 a, 26 b,26 c, 26 d including independently controlling operating speeds andthrust vectors. During hover, flight control system 32 may utilizedifferential speed control of rotor assemblies 34 a, 34 b, 34 c, 34 dfor stabilizing aircraft 10 and for providing yaw authority. This may beachieved by increasing the speed of the rotor assemblies rotatingclockwise, such as rotor assemblies 34 a, 34 c and/or decreasing thespeed of the rotor assemblies rotating counter clockwise, such as rotorassemblies 34 b, 34 d.

Alternatively or additional, flight control system 32 may utilizedifferential thrust vectoring of propulsion systems 36 a, 36 b, 36 c, 36d for stabilizing aircraft 10 and for providing yaw authority. This maybe achieved by differential longitudinal thrust vectoring of twosymmetrically disposed propulsion systems such as propulsion systems 36a, 36 c. This may also be achieved by differential thrust vectoring ofall propulsion systems 36 a, 36 b, 36 c, 36 d by suitably clocking thethrust vectors at approximately 90 degrees from one another.Alternatively or additional, flight control system 32 may utilizedifferential control surface maneuvers of control surfaces 20 a, 20 b,20 c, 20 d for stabilizing aircraft 10 and for providing yaw authority.This may be achieved by differential longitudinal control surfacemaneuvers of two symmetrically disposed control surfaces such as controlsurfaces 20 a, 20 c.

In embodiments of aircraft 10 having two-axis thrust vectoringcapabilities associated with each propulsion assembly 26 a, 26 b, 26 c,26 d, aircraft 10 has redundant direction control during hover whichserves as a safety feature in the event of a partial or complete failurein one propulsion assembly. As discussed herein, flight control system32 is operable to send commands to a symmetrically disposed propulsionassembly to counteract a thrust vector error in the compromisedpropulsion assembly. Alternatively or additional, flight control system32 is operable to send commands to any one or all of the otherpropulsion assemblies to counteract a thrust vector error in thecompromised propulsion assembly. This feature improves the overallsafety of aircraft 10 and provides redundant direction control toaircraft 10.

After vertical assent to the desired elevation, aircraft 10 may beginthe transition from thrust-borne flight to wing-borne flight. As bestseen from the progression of FIGS. 2B-2E, aircraft 10 is operable topitch forward from thrust-borne flight to wing-borne flight to enablehigh speed and/or long range forward flight. Flight control system 32may achieve this operation by increasing the speed of rotor assemblies34 c, 34 d and/or decreasing the speed of rotor assemblies 34 a, 34 b,collective thrust vectoring of propulsion systems 36 a, 36 b, 36 c, 36d, collective control surface maneuvers of control surfaces 20 a, 20 b,20 c, 20 d or any combination thereof.

As best seen in FIG. 2E, rotor assemblies 34 a, 34 b, 34 c, 34 d areeach rotating in the same vertical plane forming of a two-dimensionaldistributed thrust array. As wing-borne forward flight requiressignificantly less power then thrust-borne vertical flight, theoperating speed of some or all of propulsion assembly 26 a, 26 b, 26 c,26 d may be reduced. In certain embodiments, some of the propulsionassemblies of an aircraft of the present disclosure could be shut downduring wing-borne forward flight. In forward flight mode, theindependent control of flight control system 32 over each propulsionassembly 26 a, 26 b, 26 c, 26 d provides pitch, roll and yaw authorityusing, for example, collective or differential thrust vectoring,differential speed control, collective or differential control surfacemaneuvers or any combination thereof. In addition, as in thrust-bornevertical flight, when aircraft 10 is engaged in wing-borne forwardflight, flight control system 32 is operable to send commands to asymmetrically disposed propulsion assembly or multiple other propulsionassemblies to counteract an error in one of the propulsion assemblies.

As aircraft 10 approaches its destination, aircraft 10 may begin itstransition from wing-borne flight to thrust-borne flight. As best seenfrom the progression of FIGS. 2E-2H, aircraft 10 is operable to pitchaft from wing-borne flight to thrust-borne flight to enable, forexample, a vertical landing operation. Flight control system 32 mayachieve this operation by increasing the speed of rotor assemblies 34 a,34 b and/or decreasing the speed of rotor assemblies 34 c, 34 d,collective thrust vectoring of propulsion systems 36 a, 36 b, 36 c, 36d, collective control surface maneuvers of control surfaces 20 a, 20 b,20 c, 20 d or any combination thereof. Once aircraft 10 has completedthe transition to thrust-borne vertical flight, aircraft 10 may commenceits vertical descent to a surface. As best seen in FIG. 2I, aircraft 10has landing in a tailsitting orientation at the destination location andmay, for example, remotely drop a payload carried within pod assembly50.

Referring next to FIG. 3, the redundant directional control feature ofan aircraft 100 having a distributed thrust array including two-axisgimbal mounted propulsion systems will now be described. Aircraft 100includes a distributed thrust array depicted as four propulsionassemblies 102 a, 102 b, 102 c, 102 d forming a two-dimensional thrustarray. Propulsion assembly 102 a includes electronics node 104 a,two-axis gimbal 106 a operated by actuators 108 a, 110 a and propulsionsystem 112 a. Propulsion assembly 102 b includes electronics node 104 b,two-axis gimbal 106 b operated by actuators 108 b, 110 b and propulsionsystem 112 b. Propulsion assembly 102 c includes electronics node 104 c,two-axis gimbal 106 c operated by actuators 108 c, 110 c and propulsionsystem 112 c. Propulsion assembly 102 d includes electronics node 104 d,two-axis gimbal 106 d operated by actuators 108 d, 110 d and propulsionsystem 112 d. Each of electronics nodes 104 a, 104 b, 104 c, 104 dincludes one or more batteries, one or more controllers such as anelectronic speed controller and one or more sensors for monitoringparameters associate with the components of the respective propulsionassembly. As discussed herein, each of propulsion systems 112 a, 112 b,112 c, 112 d includes an electric motor having an output drive and arotor assembly having a plurality of rotor blades. Each rotor assemblyis rotatable with the respective output drive of the electric motor in arotational plane to generate thrust. A flight control system 114 isoperably associated with propulsion assemblies 102 a, 102 b, 102 c, 102d and is communicably linked to electronic nodes 104 a, 104 b, 104 c,104 d by communications network 116. Flight control system 114 receivessensor data from and send commands to electronic nodes 104 a, 104 b, 104c, 104 d to enable flight control system 114 to independently controleach of propulsion assemblies 102 a, 102 b, 102 c, 102 d.

For example, as best seen in FIG. 4A, aircraft 100 has longitudinalcontrol authority responsive to collective thrust vectoring ofpropulsion assemblies 102 a, 102 b, 102 c, 102 d. As illustrated,aircraft 100 has a longitudinal axis 120 and is operable for movement inthe longitudinal direction as indicated by arrow 122. In the illustratedembodiment, flight control system 114 has sent commands to operate eachof actuators 108 a, 108 b, 108 c, 108 d to tilt each of propulsionsystems 112 a, 112 b, 112 c, 112 d in the forward direction. Actuators110 a, 110 b, 110 c, 110 d are in an unactuated state. In thisconfiguration, propulsion assemblies 102 a, 102 b, 102 c, 102 d generatethrust vectors having aftward directed longitudinal components 124 a,124 b, 124 c, 124 d. In hover, such collective thrust vectoring ofpropulsion assemblies 102 a, 102 b, 102 c, 102 d provides longitudinalcontrol authority to aircraft 100.

The longitudinal thrust vectoring operation will now be described withreference to an exemplary propulsion assembly 102, depicted as a linereplaceable propulsion unit, in FIGS. 5A-5C. Propulsion assembly 102includes a housing 126 and a gimbal 106 that is coupled to housing 126.Gimbal 106 includes an outer gimbal member 128 and an inner gimbalmember 130. Outer gimbal member 128 is pivotally coupled to housing 126and is operable to tilt about a first axis. Inner gimbal member 130 ispivotally coupled to outer gimbal member 128 and is operable to tiltabout a second axis that is orthogonal to the first axis. In theillustrated embodiment, actuator 108 is coupled between housing 126 andouter gimbal member 128 such that operation of actuator 108 shiftlinkage 132 to tilt outer gimbal member 128 about the first axisrelative to housing 126. Actuator 110 is coupled between housing 126 andinner gimbal member 130 such that operation of actuator 110 shiftslinkage 134 to tilt inner gimbal member 130 about the second axisrelative to outer gimbal member 128 and housing 126. A propulsion system112 is coupled to and is operable to tilt with gimbal 106 about bothaxes relative to housing 126. In the illustrated embodiment, the rotorassembly has been removed from propulsion system 112 such that onlyelectric motor 136 and output drive 138 are visible in the figures.

As best seen in the comparison of FIGS. 5A-5C, actuator 108 is operatedto tilt propulsion system 112 longitudinally between a fully forwardconfiguration shown in FIG. 5A and a fully aft configuration shown inFIG. 5C as well as in an infinite number of positions therebetweenincluding the fully vertical configuration shown in FIG. 5B. Thisoperation longitudinally shifts the thrust vector of propulsion assembly102 to enable the longitudinal control authority of aircraft 100depicted in FIG. 4A. The maximum longitudinal tilt angle of gimbal 106may preferably be between about 10 degrees and about 30 degrees, maymore preferably be between about 15 degrees and about 25 degrees and maymost preferably be about 20 degrees. As should be understood by thosehaving ordinary skill in the art, the magnitude of the longitudinalcomponent 124 of the thrust vector is related to the direction of thethrust vector, which is determined by the longitudinal tilt angle ofgimbal 106.

As best seen in FIG. 4B, aircraft 100 has lateral control authorityresponsive to collective thrust vectoring of propulsion assemblies 102a, 102 b, 102 c, 102 d. As illustrated, aircraft 100 has a longitudinalaxis 120 and is operable for movement in the lateral direction asindicated by arrow 142. In the illustrated embodiment, flight controlsystem 114 has sent commands to operate each of actuators 110 a, 110 b,110 c, 110 d to tilt each of propulsion systems 112 a, 112 b, 112 c, 112d to the right (from a forward looking perceptive from longitudinal axis120). Actuators 108 a, 108 b, 108 c, 108 d are in an unactuated state.In this configuration, propulsion assemblies 102 a, 102 b, 102 c, 102 dgenerate thrust vectors having leftwardly directed lateral components144 a, 144 b, 144 c, 144 d. In hover, such collective thrust vectoringof propulsion assemblies 102 a, 102 b, 102 c, 102 d provides lateralcontrol authority to aircraft 100.

The lateral thrust vectoring operation will now be described withreference to propulsion assembly 102 in FIGS. 5D-5F. As best seen in thecomparison of FIGS. 5D-5F, actuator 110 is operated to tilt propulsionsystem 112 lateral between a fully right configuration shown in FIG. 5Dand a fully left configuration shown in FIG. 5F as well as in aninfinite number of positions therebetween including the fully verticalconfiguration shown in FIG. 5E. This operation laterally shifts thethrust vector of propulsion assembly 102 to enable the lateral controlauthority of aircraft 100 depicted in FIG. 4B. The maximum lateral tiltangle of gimbal 106 may preferably be between about 10 degrees and about30 degrees, may more preferably be between about 15 degrees and about 25degrees and may most preferably be about 20 degrees. As should beunderstood by those having ordinary skill in the art, the magnitude ofthe lateral component 144 of the thrust vector is related to thedirection of the thrust vector, which is determined by the lateral tiltangle of gimbal 106.

Using both the longitudinal and lateral control authority provided bycollective thrust vectoring of propulsion assemblies 102 a, 102 b, 102c, 102 d, provides omnidirectional horizontal control authority foraircraft 100. For example, as best seen in FIG. 4C, aircraft 100 hasdiagonal control authority responsive to collective thrust vectoring ofpropulsion assemblies 102 a, 102 b, 102 c, 102 d. As illustrated,aircraft 100 has a longitudinal axis 120 and is operable for movement inthe diagonal direction as indicated by arrow 152. In the illustratedembodiment, flight control system 114 has sent commands to operate eachof actuators 108 a, 108 b, 108 c, 108 d and actuators 110 a, 110 b, 110c, 110 d to tilt each of propulsion systems 112 a, 112 b, 112 c, 112 dforward/right. In this configuration, propulsion assemblies 102 a, 102b, 102 c, 102 d generate thrust vectors having aft/leftward directedcomponents 154 a, 154 b, 154 c, 154 d. In hover, such collective thrustvectoring of propulsion assemblies 102 a, 102 b, 102 c, 102 d providesdiagonal control authority to aircraft 100.

The diagonal thrust vectoring operation will now be described withreference to propulsion assembly 102 in FIGS. 5G-5I. As best seen in thecomparison of FIGS. 5G-5I, actuators 108, 110 are operated to tiltpropulsion system 112 diagonally between a fully aft/right configurationshown in FIG. 5G and a fully forward/left configuration shown in FIG. 5Ias well as in an infinite number of positions therebetween including thefully vertical configuration shown in FIG. 5H. This operation diagonallyshifts the thrust vector of propulsion assembly 102 to enable thediagonal control authority of aircraft 100 depicted in FIG. 4C. Themaximum diagonal tilt angle of gimbal 106 may preferably be betweenabout 10 degrees and about 30 degrees, may more preferably be betweenabout 15 degrees and about 25 degrees and may most preferably be about20 degrees. As should be understood by those having ordinary skill inthe art, the magnitude of the diagonal component 154 of the thrustvector is related to the direction of the thrust vector, which isdetermined by the diagonal tilt angle of gimbal 106.

In addition to collective thrust vectoring of propulsion assemblies 102a, 102 b, 102 c, 102 d, aircraft 100 is also operable to engage indifferential thrust vectoring of propulsion assemblies 102 a, 102 b, 102c, 102 d. For example, as best seen in FIG. 4D, aircraft 100 has yawauthority responsive to differential thrust vectoring of propulsionassemblies 102 a, 102 b, 102 c, 102 d. As illustrated, aircraft 100 hasa longitudinal axis 120 and is operable for rotation thereabout asindicated by arrow 162. In the illustrated embodiment, flight controlsystem 114 has sent commands to operate each of actuators 108 a, 108 b,108 c, 108 d and actuators 110 a, 110 b, 110 c, 110 d to tilt propulsionsystem 112 a forward/right, to tilt propulsion system 112 b aft/right,to tilt propulsion system 112 c aft/left and to tilt propulsion system112 d forward/left. In this configuration, propulsion assemblies 102 a,102 b, 102 c, 102 d generate thrust vectors having horizontal components164 a, 164 b, 164 c, 164 d. In hover, such differential thrust vectoringof propulsion assemblies 102 a, 102 b, 102 c, 102 d provides yawauthority to aircraft 100.

As discussed herein, outer gimbal member 128 is pivotally coupled tohousing 126 and is operable to tilt about the first axis and innergimbal member 130 is pivotally coupled to outer gimbal member 128 and isoperable to tilt about the second axis that is orthogonal to the firstaxis. In the illustrated embodiment, in order to minimize the energyrequired to tilt propulsion system 112 relative to housing 126 to changethe thrust vector direction of propulsion assembly 102, the first andsecond axes pass through propulsion system 112. The precise location ofthe intersection of the axes through propulsion system 112 may bedetermined based on factors including the mass of propulsion system 112,the size and shape of propulsion system 112, the desired rotationalvelocity of propulsion system 112 during thrust vectoring and otherfactors that should be understood by those having ordinary skill in theart. In one implementation, the first and second axes may pass throughthe center of mass of propulsion system 112. Alternatively, it may bedesirable to have the first and second axes pass through a location nearthe center of mass of propulsion system 112 such as within apredetermined distance from the center of mass of propulsion system 112.The predetermined distance may be selected based upon criteria such as adefined volume surrounding the center of mass that contains apredetermined portion of the total mass of propulsion system 112. Forexample, the first and second axes may pass through a location within avolume centered at the center of mass of propulsion system 112 thatcontains no more than ten percent of the mass of propulsion system 112.Such a volume may be expressed, for example, as being within onecentimeter, one inch or other predetermined distance from the center ofmass of propulsion system 112.

Due to dynamic effects caused by the rotation of the rotor assembly andthe lift generated by the rotor assembly during flight operations, suchas during thrust-borne flight operations, the center of mass in hover ofpropulsion system 112 may not coincide with the actual center of mass ofpropulsion system 112. To compensate for the dynamic effects, the firstand second axes may pass through the center of mass in hover ofpropulsion system 112. Alternatively, it may be desirable to have thefirst and second axes pass through a location near the center of mass inhover of propulsion system 112 such as within a predetermined distancefrom the center of mass in hover of propulsion system 112. In oneexample, it may be desirable to have the first and second axes passthrough a location between the center of mass of propulsion system 112and the center of mass in hover of propulsion system 112.

Referring now to FIGS. 6A-6D, the redundant directional control featureof aircraft 100 will now be described. In the illustrated embodiment,aircraft 100 includes a distributed thrust array depicted as four ofpropulsion assemblies 102 a, 102 b, 102 c, 102 d that form atwo-dimensional thrust array. As discussed herein and as best seen inFIG. 3, each propulsion assembly 102 a, 102 b, 102 c, 102 d includes anelectronics node, a two-axis gimbal operated by two independentactuators and a propulsion system 112 a, 112 b, 112 c, 112 d that isoperable to tilt with the gimbal relative to the propulsion assemblyhousing and the airframe of aircraft 100. Flight control system 114 isoperable to independently control the operating speeds of each electricmotor and is operable to independently control the positions of eachactuator such that for each propulsion assembly 102 a, 102 b, 102 c, 102d, a thrust vector can be resolved within a thrust vector cone.Importantly, in the event of an actuator fault or other fault in one ofthe propulsion assemblies, flight control system 114 sends commands toat least the symmetrically disposed propulsion assembly to counteractthe fault. For example, to overcome a thrust vector error in one of thepropulsion assemblies, flight control system 114 autonomously engages incorrective operations such as adjusting the thrust vector of thesymmetrically disposed propulsion assembly to counteract the thrustvector error. Adjusting the thrust vector of the symmetrically disposedpropulsion system may include tilting the propulsion system about thefirst axis, tilting the propulsion system about the second axis,changing the operating speed of the electric motor and combinationsthereof. This autonomous corrective operation capability serves asredundancy in the directional control of aircraft 100 allowing aircraft100 to have flight control in hover even during fault conditions.

Referring specifically to FIG. 6A, a thrust vector error in propulsionassembly 102 b has occurred due to, for example, a static actuator faultcausing propulsion system 112 b of propulsion assembly 102 b to ceasetilting in the longitudinal direction. The thrust vector error isdepicted as dashed arrow 170. Flight control system 114 recognizes thethrust vector error of propulsion assembly 102 b and sends commands toat least propulsion assembly 102 d to counteract the single-axis staticactuator fault in propulsion assembly 102 b. In this case, the commandsmay include shifting actuator 108 d to adjust the thrust vector ofpropulsion assembly 102 d to include a corrective component depicted assolid arrow 172 that maintains the stability of aircraft 100. Inaddition, flight control system 114 may command propulsion assemblies102 a, 102 c to perform addition corrective actions to assist incounteracting the thrust vector error of propulsion assembly 102 b. Evenin the fault condition, as propulsion assembly 102 b continues toprovide significant thrust in the vertical direction and thrust vectorcapability in the lateral direction, it may be desirable to maintain theoperation of propulsion assembly 102 b until aircraft 100 makes a safelanding allowing the autonomous corrective actions of flight controlsystem 114 to counteract the thrust vector error.

Referring specifically to FIG. 6B, a thrust vector error in propulsionassembly 102 c has occurred due to, for example, a static actuator faultcausing propulsion system 112 c of propulsion assembly 102 c to ceasetilting in both the longitudinal and lateral directions. The thrustvector error is depicted as dashed arrow 174. Flight control system 114recognizes the thrust vector error of propulsion assembly 102 c andsends commands to at least propulsion assembly 102 a to counteract thetwo-axis static actuator fault in propulsion assembly 102 c. In thiscase, the commands may include shifting actuators 108 a, 110 a to adjustthe thrust vector of propulsion assembly 102 a to include a correctivecomponent depicted as solid arrow 176 that maintains the stability ofaircraft 100. In addition, flight control system 114 may commandpropulsion assemblies 102 b, 102 d to perform addition correctiveactions to assist in counteracting the thrust vector error of propulsionassembly 102 c. Even in the fault condition, as propulsion assembly 102c continues to provide significant thrust in the vertical direction, itmay be desirable to maintain the operation of propulsion assembly 102 cuntil aircraft 100 makes a safe landing allowing the autonomouscorrective actions of flight control system 114 to counteract the thrustvector error.

Referring specifically to FIG. 6C, a thrust vector error in propulsionassembly 102 c has occurred due to, for example, a single-axis dynamicactuator fault causing propulsion system 112 c of propulsion assembly102 b to tilt uncontrolled in the lateral direction. The thrust vectorerror is depicted as dashed arrows 178 a, 178 b that represent acontinuum between maximum error positions. Flight control system 114recognizes the thrust vector error of propulsion assembly 102 c andsends commands to at least propulsion assembly 102 a to counteract thesingle-axis dynamic actuator fault in propulsion assembly 102 c. In thiscase, the commands may include continually shifting actuator 110 a todynamically adjust the thrust vector of propulsion assembly 102 a toinclude the time dependent corrective component depicted as solid arrows180 a, 180 b representing the continuum between maximum correctivepositions. The corrective action maintains the stability of aircraft100. In addition, flight control system 114 may command propulsionassemblies 102 b, 102 d to perform addition corrective actions to assistin counteracting the thrust vector error of propulsion assembly 102 c.Even in the fault condition, as propulsion assembly 102 c continues toprovide significant thrust in the vertical direction, it may bedesirable to maintain the operation of propulsion assembly 102 c untilaircraft 100 makes a safe landing allowing the autonomous correctiveactions of flight control system 114 to counteract the thrust vectorerror.

Referring specifically to FIG. 6D, a thrust vector error in propulsionassembly 102 d has occurred due to, for example, a two-axis dynamicactuator fault causing propulsion system 112 d of propulsion assembly102 d to tilt uncontrolled in the longitudinal and lateral directions.The thrust vector error is depicted as dashed arrows 182 a, 182 b, 182c, 182 d within dashed circle 184 representing the universe of errorpositions. Flight control system 114 recognizes the thrust vector errorof propulsion assembly 102 d and sends commands to at least propulsionassembly 102 b to counteract the two-axis dynamic actuator fault inpropulsion assembly 102 d. In this case, the commands may includecontinually shifting actuators 108 b, 110 b to dynamically adjust thethrust vector of propulsion assembly 102 b to include the time dependentcorrective component depicted as solid arrows 186 a, 186 b, 186 c, 186 dwithin solid circle 188 representing the universe of correctivepositions. The corrective action maintains the stability of aircraft100. In addition, flight control system 114 may command propulsionassemblies 102 a, 102 c to perform addition corrective actions to assistin counteracting the thrust vector error of propulsion assembly 102 b.Even in the fault condition, as propulsion assembly 102 d continues toprovide significant thrust in the vertical direction, it may bedesirable to maintain the operation of propulsion assembly 102 d untilaircraft 100 makes a safe landing allowing the autonomous correctiveactions of flight control system 114 to counteract the thrust vectorerror.

In addition to performing autonomous corrective actions to counteract athrust vector error, flight control system 114 may autonomously commandaircraft 100 to perform other flight maneuvers. Depending upon the typeof fault and the magnitude of the thrust vector error caused by thefault, flight control system 114 may command aircraft 100 to return to amaintenance center or other predetermined location. Under other faultsituations, flight control system 114 may command aircraft 100 toinitiate a jettison sequence of the pod assembly or other payload and/orperform an emergency landing. If the fault is not critical and/or issuitably overcome by the corrective actions described herein, flightcontrol system 114 may command aircraft 100 to continue the currentmission. In this case, flight control system 114 may command aircraft100 to adjust the center of mass of the pod assembly or other payloadrelative to the airframe such as lowering the elevation of the podassembly relative to the airframe as this may improve hover stability.

Referring next to FIG. 7, the directional control of an aircraft 200having a distributed thrust array including single-axis gimbal mountedpropulsion systems will now be described. Aircraft 200 includes adistributed thrust array depicted as four of propulsion assemblies 202a, 202 b, 202 c, 202 d that form a two-dimensional thrust array.Propulsion assembly 202 a includes electronics node 204 a, single-axisgimbal 206 a operated by actuator 208 a and propulsion system 112 a.Propulsion assembly 202 b includes electronics node 204 b, single-axisgimbal 206 b operated by actuator 208 b and propulsion system 112 b.Propulsion assembly 202 c includes electronics node 204 c, single-axisgimbal 206 c operated by actuator 208 c and propulsion system 112 c.Propulsion assembly 202 d includes electronics node 204 d, single-axisgimbal 206 d operated by actuator 208 d and propulsion system 112 d.Each of electronics nodes 204 a, 204 b, 204 c, 204 d includes one ormore batteries and one or more controllers such as an electronic speedcontroller. As discussed herein, each of propulsion systems 202 a, 202b, 202 c, 202 d includes an electric motor having an output drive and arotor assembly having a plurality of rotor blades. Each rotor assemblyis rotatable with the respective output drive of the electric motor in arotational plane to generate thrust. A flight control system 214 isoperably associated with propulsion assemblies 202 a, 202 b, 202 c, 202d and is communicably linked to electronic nodes 204 a, 204 b, 204 c,204 d by communications network 216. Flight control system 214 sendcommands to electronic nodes 204 a, 204 b, 204 c, 204 d to enable flightcontrol system 214 to independently control each of propulsionassemblies 202 a, 202 b, 202 c, 202 d.

For example, as best seen in FIG. 8A, aircraft 200 has longitudinalcontrol authority responsive to collective thrust vectoring ofpropulsion assemblies 202 a, 202 b, 202 c, 202 d. As illustrated,aircraft 200 has a longitudinal axis 220 and is operable for movement inthe longitudinal direction as indicated by arrow 222. Flight controlsystem 214 has sent commands to operate each of actuators 208 a, 208 b,208 c, 208 d to tilt each of propulsion systems 212 a, 212 b, 212 c, 212d in the forward direction. In this configuration, propulsion assemblies202 a, 202 b, 202 c, 202 d generate thrust vectors having aftwarddirected longitudinal components 224 a, 224 b, 224 c, 224 d. In hover,such collective thrust vectoring of propulsion assemblies 202 a, 202 b,202 c, 202 d provides longitudinal control authority to aircraft 200.

The longitudinal thrust vectoring operation will now be described withreference to an exemplary propulsion assembly 202, depicted as a linereplaceable propulsion unit, in FIGS. 9A-9C. Propulsion assembly 202includes a housing 226 and a gimbal 206 that is pivotally coupled tohousing 126 and is operable to tilt about a single axis. In theillustrated embodiment, actuator 208 is coupled between housing 226 andgimbal 206 such that operation of actuator 208 shifts linkage 232 totilt gimbal 206 about the axis relative to housing 226. A propulsionsystem 212 is coupled to and is operable to tilt with gimbal 206 aboutthe axis relative to housing 226. In the illustrated embodiment, therotor assembly has been removed from propulsion system 212 such thatonly electric motor 236 and output drive 238 are visible in the figures.

As best seen in the comparison of FIGS. 9A-9C, actuator 208 is operatedto tilt propulsion system 212 longitudinally between a fully forwardconfiguration shown in FIG. 9A and a fully aft configuration shown inFIG. 9C as well as in an infinite number of positions therebetweenincluding the fully vertical configuration shown in FIG. 9B. Thisoperation longitudinally shifts the thrust vector of propulsion assembly202 to enable the longitudinal control authority of aircraft 200depicted in FIG. 8A. The maximum longitudinal tilt angle of gimbal 206may preferably be between about 10 degrees and about 30 degrees, maymore preferably be between about 15 degrees and about 25 degrees and maymost preferably be about 20 degrees. As should be understood by thosehaving ordinary skill in the art, the magnitude of the longitudinalcomponent 224 of the thrust vector is related to the direction of thethrust vector, which is determined by the longitudinal tilt angle ofgimbal 206.

In the illustrated embodiment, the single gimbal axis is located belowpropulsion system 212. In other single gimbal axis embodiments andsimilar to propulsion assembly 102 of FIGS. 5A-5I, the single gimbalaxis could alternately pass through propulsion system 212. For example,the single gimbal axis could pass through the center of mass ofpropulsion system 212 or through a location near the center of mass ofpropulsion system 212, such as within a predetermined distance from thecenter of mass of propulsion system 212. As another example, the singlegimbal axis could pass through the center of mass in hover of propulsionsystem 212, through a location near the center of mass in hover ofpropulsion system 112, such as within a predetermined distance from thecenter of mass in hover of propulsion system 112, or through a locationbetween the center of mass of propulsion system 212 and the center ofmass in hover of propulsion system 212.

In addition to collective thrust vectoring of propulsion assemblies 202a, 202 b, 202 c, 202 d, aircraft 200 is also operable to engage indifferential longitudinal thrust vectoring of propulsion assemblies 202a, 202 b, 202 c, 202 d. For example, as best seen in FIG. 8B, aircraft200 has yaw authority responsive to differential longitudinal thrustvectoring of propulsion assemblies 202 b, 202 d. As illustrated,aircraft 200 has a longitudinal axis 220 and is operable for rotationthereabout as indicated by arrow 226. Flight control system 214 has sentcommands to operate actuator 208 b to tilt propulsion system 212 bforward and to operate actuator 208 d to tilt propulsion system 212 daftward. In this configuration, propulsion assembly 212 b generates athrust vector having an aftward directed longitudinal component 228 band propulsion assembly 212 d generates a thrust vector having a forwarddirected longitudinal component 228 d. In hover, such differentiallongitudinal thrust vectoring of symmetrically disposed propulsionassemblies, such as propulsion assemblies 202 b, 202 d, provides yawauthority to aircraft 200.

Referring to FIGS. 10A-10D, various independent mechanisms for providingyaw authority in hover to an aircraft of the present disclosure will nowbe described. Aircraft 10 described above will be used as the exampleaircraft for the present discussion wherein aircraft 10 would includefour of the illustrated tail sections. Each tail section includes a tailmember 300 depicted with a propulsion assembly 302 and a control surface304 coupled thereto. Propulsion assembly 302 includes a rotor assembly306 and may represent any propulsion assembly discussed herein includingpropulsion assemblies operable for single-axis thrust vectoring,two-axis thrust vectoring or no thrust vectoring. Control surface 304 isan active control surface operable for tilting in the longitudinaldirection of aircraft 10 by actuator 308 via linkage 310 responsive tocommands from flight control system 32. In the illustrated embodiment,rotor assembly 306 has a rotor diameter D and control surface 304 isless than two rotor diameters (2D) and preferably between one rotordiameter and two rotor diameters from rotor assembly 306. Locatingcontrol surface 304 within the specified distance from rotor assembly306 enables control surface 304 to operate in the propwash of rotorassembly 306 in both thrust-borne flight and wing-borne flight.

If aircraft 10 utilizes embodiments of propulsion assembly 302 with nothrust vectoring, aircraft 10 has two independent yaw authoritymechanisms in hover. In one approach, differential speed control is usedto change the relative rotor speeds of the rotor assemblies rotatingclockwise compared to the rotor assemblies rotating counterclockwisecausing a torque imbalance in aircraft 10, which provides yaw authority.This operation may be represented by the tail section configuration inFIG. 10A. In the other approach, differential longitudinal controlsurface maneuvers of control surfaces 304 of two symmetrically disposedtail sections are used to create a yaw moment responsive to propwashblowing over the tilted control surfaces 304. This operation may berepresented by the tail section configuration in FIG. 10C. Dependingupon the yaw authority requirement, it may be desirable to use one yawauthority mechanism instead of another due to factors such as theresponse rate and yaw moment of a particular yaw authority mechanism.For example, a yaw authority mechanism with a faster response rate maybe preferred for small and/or continuous corrections while a yawauthority mechanism with a larger yaw moment may be preferred for largecorrection and/or certain aircraft maneuvers such as large rotationsabout the longitudinal axis. In addition, an aircraft 10 having nonthrust vectoring propulsion assemblies 302 may use a combination ofdifferential speed control and differential longitudinal control surfacemaneuvers to provide yaw authority. This operation may be represented bythe tail section configuration in FIG. 10C.

If aircraft 10 utilizes embodiments of propulsion assembly 302 havingsingle-axis or two-axis thrust vectoring, aircraft 10 has threeindependent yaw authority mechanisms in hover. In one approach,differential speed control is used to change the relative rotor speedsof the rotor assemblies rotating clockwise compared to the rotorassemblies rotating counterclockwise causing a torque imbalance inaircraft 10, which provides yaw authority. This operation may berepresented by the tail section configuration in FIG. 10A. In anotherapproach, differential longitudinal control surface maneuvers of controlsurfaces 304 of two symmetrically disposed tail sections are used tocreate a yaw moment responsive to propwash blowing over the tiltedcontrol surfaces 304. This operation may be represented by the tailsection configuration in FIG. 10C. In the next approach, differentialthrust vectoring is used to generate a yaw moment. In either single ortwo-axis thrust vectoring embodiments, this may be achieved bydifferential longitudinal thrust vectoring of two symmetrically disposedpropulsion systems (see FIG. 8B). In addition, in two-axis thrustvectoring embodiments, this may be achieved by differential thrustvectoring of all propulsion systems by suitably clocking the thrustvectors at approximately 90 degrees from one another (see FIG. 4D). Thisoperation may be represented by the tail section configuration in FIG.10B. Depending upon the yaw authority requirement, it may be desirableto use a faster response rate yaw authority mechanism for small and/orcontinuous corrections and a yaw authority mechanism with a larger yawmoment for large correction and/or certain aircraft maneuvers.

In addition, an aircraft 10 having thrust vectoring propulsionassemblies 302 may use a combination of differential speed control,differential longitudinal control surface maneuvers and differentialthrust vectoring to provide yaw authority. For example, aircraft 10could utilize differential speed control in combination withdifferential longitudinal control surface maneuvers, which may berepresented by the tail section configuration in FIG. 10C. As anotherexample, aircraft 10 could utilize differential speed control incombination with differential thrust vectoring, which may be representedby the tail section configuration in FIG. 10B. As a further example,aircraft 10 could utilize differential longitudinal control surfacemaneuvers in combination with differential thrust vectoring, which maybe represented by the tail section configuration in FIG. 10D. In a finalexample, aircraft 10 could utilize differential speed control incombination with differential longitudinal control surface maneuvers anddifferential thrust vectoring, which may be represented by the tailsection configuration in FIG. 10D.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. An aircraft having redundant directional control,the aircraft comprising: an airframe; a two-dimensional distributedthrust array attached to the airframe, the thrust array including aplurality of propulsion assemblies; and a flight control system operableto independently control each of the propulsion assemblies; eachpropulsion assembly comprising: a housing; a gimbal coupled to thehousing and operable to tilt about first and second axes; first andsecond actuators operable to tilt the gimbal respectively about thefirst and second axes; and a propulsion system coupled to and operableto tilt with the gimbal, the propulsion system including an electricmotor having an output drive and a rotor assembly having a plurality ofrotor blades, the rotor assembly rotatable with the output drive of theelectric motor in a rotational plane to generate thrust having a thrustvector; wherein, responsive to a thrust vector error of a firstpropulsion assembly, the flight control system commands at least asecond propulsion assembly that is symmetrically disposed relative tothe first propulsion assembly to counteract the thrust vector error,thereby providing redundant directional control for the aircraft.
 2. Theaircraft as recited in claim 1 wherein the flight control system furthercomprises a redundant flight control system.
 3. The aircraft as recitedin claim 1 wherein the flight control system further comprises a triplyredundant flight control system.
 4. The aircraft as recited in claim 1wherein the commands of the flight control system to the secondpropulsion assembly are selected from the group consisting of tiltingthe second propulsion assembly about the first axis, tilting the secondpropulsion assembly about the second axis, changing an operating speedof the rotor assembly of the second propulsion assembly and combinationsthereof.
 5. The aircraft as recited in claim 1 wherein the thrust vectorerror of the first propulsion assembly further comprises an actuatorfault in the first propulsion assembly.
 6. The aircraft as recited inclaim 5 wherein the actuator fault in the first propulsion assemblyfurther comprises a static actuator fault causing the propulsion systemof the first propulsion assembly to cease tilting about one axis andwherein the flight control system commands the second propulsionassembly to counteract the single axis static actuator fault.
 7. Theaircraft as recited in claim 5 wherein the actuator fault in the firstpropulsion assembly further comprises a static actuator fault causingthe propulsion system of the first propulsion assembly to cease tiltingabout both axes and wherein the flight control system commands thesecond propulsion assembly to counteract the two-axis static actuatorfault.
 8. The aircraft as recited in claim 5 wherein the actuator faultin the first propulsion assembly further comprises a dynamic actuatorfault causing the propulsion system of the first propulsion assembly totilt uncontrolled about one axis and wherein the flight control systemcommands the second propulsion assembly to counteract the single axisdynamic actuator fault.
 9. The aircraft as recited in claim 5 whereinthe actuator fault in the first propulsion assembly further comprises adynamic actuator fault causing the propulsion system of the firstpropulsion assembly to tilt uncontrolled about both axes and wherein theflight control system commands the second propulsion assembly tocounteract the two-axis dynamic actuator fault.
 10. The aircraft asrecited in claim 1 wherein, responsive to the thrust vector error of thefirst propulsion assembly, the flight control system commands at leasttwo other propulsion assemblies to counteract the thrust vector error,thereby providing redundant directional control for the aircraft. 11.The aircraft as recited in claim 1 wherein, responsive to the thrustvector error of the first propulsion assembly, the flight control systemcommands the aircraft to land at a predetermined location.
 12. Theaircraft as recited in claim 1 wherein, responsive to the thrust vectorerror of the first propulsion assembly, the flight control systemcommands the aircraft to perform an emergency landing.
 13. The aircraftas recited in claim 1 wherein, responsive to the thrust vector error ofthe first propulsion assembly, the flight control system commands theaircraft to continue a current mission.
 14. The aircraft as recited inclaim 1 wherein, responsive to the thrust vector error of the firstpropulsion assembly, the flight control system commands the aircraft toadjust a center of mass of a payload relative to the airframe.
 15. Theaircraft as recited in claim 1 wherein, responsive to the thrust vectorerror of the first propulsion assembly, the flight control systemcommands the aircraft to initiate a jettison sequence.
 16. The aircraftas recited in claim 1 wherein the plurality of propulsion assembliesfurther comprises at least four propulsion assemblies forming thetwo-dimensional thrust array.
 17. The aircraft as recited in claim 1further comprising a thrust-borne flight mode and a wing-borne flightmode.
 18. The aircraft as recited in claim 1 wherein the airframefurther comprises first and second wings having at least first andsecond pylons extending therebetween and having a plurality of tailmembers extending therefrom, each tail member having a control surface.19. The aircraft as recited in claim 1 further comprising a pod assemblycoupled to the airframe.
 20. An aircraft having a thrust-borne flightmode and a wing-borne flight mode, the aircraft comprising: an airframehaving first and second wings with at least first and second pylonsextending therebetween and with a plurality of tail members extendingtherefrom; a pod assembly coupled to the airframe between the first andsecond pylons; a two-dimensional distributed thrust array attached tothe airframe, the thrust array including at least four line replaceablepropulsion units; and a flight control system operable to independentlycontrol each of the propulsion units; each propulsion unit comprising: ahousing; a gimbal coupled to the housing and operable to tilt aboutfirst and second axes; first and second actuators operable to tilt thegimbal respectively about the first and second axes; and a propulsionsystem coupled to and operable to tilt with the gimbal, the propulsionsystem including an electric motor having an output drive and a rotorassembly having a plurality of rotor blades, the rotor assemblyrotatable with the output drive of the electric motor in a rotationalplane to generate thrust having a thrust vector; wherein, responsive toa thrust vector error of a first propulsion assembly, the flight controlsystem commands at least a second propulsion assembly that issymmetrically disposed relative to the first propulsion assembly tocounteract the thrust vector error, thereby providing redundantdirectional control for the aircraft.